Data transfer systems for transferring data files among data processing stations, such as general purpose computers, are well known. Data transfer network exists wherein a number of computers are interconnected and each computer is a node in the network. Data files are transferred through the network using established network protocols. In such networks the separate computer installations may provide a variety of computing services at the various nodes of the network and the interconnecting network links make it possible for one computer system to use the services of another. This kind of interaction is customarily accompanied by an extensive transfer of data files between computer installations. File transfers are initiated in response to user requests which can be handled only when the data transfer resources are available. Each node in the network has one or more data transfer facilities which form the network interface at the node. These facilities include hardware and software and the number of active facilities affects network congestion and affects the availability of computer resources at each of the nodes. The network links are expensive and increasingly in demand as the network grows. The transfer facilities also are expensive and their activation requires the use of computer resources in demand for other purposes.
In order to transfer a data file from one node to another, the sending node must have a transfer facility available for transmission and the receiving node must have a transfer facility available to receive the file. It may well be that a transfer facility is not available at the receiving node at the time that an attempt to transfer is made by the sending station. In that case, the attempt will be unsuccessful and the transfer facility which was available at the transmitting end may be used to transmit files to a different destination. Hence, it may not be available when facilities at the original destination become free. It can be seen, that with such an approach it is possible that some files will be delayed in their transfer a much longer period of time than other files. In fact, it is not unusual to find that files destined for certain destinations consistently encounter long delays.
In some networks the unavailability of facilities at the receiving end is detected only by setting up a connection and testing for availability. Unsuccessful transfer attempts unnecessarily add to network congestion and may interfere with other transfers. This adds to the expense of the network and degrades network performance.
It is an object of this invention to make optimum use of the network transfer facilities and to avoid unnecessary network congestion.